FIG. 1 is a sectional view showing a conventional cryostat. As illustrated, liquid cryogen, such as liquid helium 1, which is a liquefied gas, is contained in a cryogen container 2 accommodating a superconducting magnet including a superconducting coil 10 wound in the interior of the cryogen container 2. A helium gas 3, which results from evaporation of the liquid helium, is in the helium gas container 2, and is staying above the liquid surface. A heat shield (radiation shield) 4 is provided to surround the cryogen container 2. A vacuum container 5 is provided to surround the heat shield 4 and maintain its interior in a vacuum state. A refrigerator system 6 is provided for cooling the heat shield 4 and recondensing the helium gas 3 in the cryogen container 2. The refrigerator system 6 comprises a refrigerator unit 7 and a compressor unit 8. The refrigerator unit 7 has a main block 7a situated outside the vacuum container 5, an elongated, e.g., cylindrical part 7b which extends through the walls of the vacuum container 5 and the heat shield 4 having first-stage and second-stage cooling sections 7c and 7d which are disposed near the walls of the heat shield 4 and the cryogen container 2 and thermally connected therewith for cooling the heat shield 4 and the cryogen container 2, respectively.
The operation will next be described. The liquid helium 1 cools the superconducting magnet. The heat shield 4 reduces infiltration of heat from outside to inside of the cryogen container 2. The surrounding vacuum container 2 further gives vacuum heat insulation. But there is still some infiltration of heat, and, for this reason, the liquid helium evaporates to become the helium gas 3. The refrigerator system 6 recondenses the helium gas to restrain reduction in the amount of the liquid helium 1.
A problem associated with the conventional cryostat configured as described above is that when the cooling by the refrigerator is excessive and the condensation of the evaporated gas proceeds excessively, the interior of the container containing the liquid gas may be of a negative pressure, and air may be drawn into the container from a tube extending to the exterior. Also, due to the variation in the interior pressure, the container 2 may be deformed, and, the superconducting coil 10 wound on the inner wall surface of the cryogen container 2 may also be deformed, and the magnetic field strength and the magnetic filed uniformity may be affected.